1. Field of the Invention
This invention is directed to plastic clad optical fibers and their method of production.
2. Description of the Prior Art
Efficient transmission of information through long optical waveguides is dependent on the exceptional optical clarity of existing optical fibers. Technological advances during the 1970's have reduced attenuation in optical fibers to approximately 3 dB/km at 0.8 .mu.m wavelength. However, studies have shown that when these fibers are exposed to ionizing radiation, attenuation increases substantially. The loss is time dependent to the extent that a krad of radiation (3 nsec) increases the loss from 3 dB/km to several thousand dB/km for short time periods on the order of 1 msec or less. The loss rapidly decreases with increasing time after exposure to a residual loss of 30 to 200 dB/km, which may be permanent.
Although there is a relatively wide variation in radiation sensitivity of currently marketed optical fibers, the previously described attenuation behavior is typical of state-of-the-art fibers. This behavior poses a significant problem in designing optical fibers suitable for use in environments where there is a potential for exposure to radiation, particularly nuclear threat environments.
Recent work by Friebele et al, reported in Appl. Phys. Lett., 32, 95 (1978), demonstrates that high dose levels of radiation, in the range of 10.sup.6 rads, on silicone resin causes only a doubling of the initial loss, i.e., a 3 dB increase. These findings suggest that a silicone clad-silica core optical fiber would exhibit high radiation resistance. The very recent paper by G. H. Sigel et al titled "Radiation Response of Large Core Polymer Clad Silica Optical Fibers" presented at the IEEE Nuclear and Space Radiation Effects Conference (Santa Cruz, CA July 17-20, 1979) presented experimental confimation that plastic clad silica fibers are indeed the most radiation resistant optical waveguides currently available.
Unfortunately, the use of a silicone cladding in such an optical fiber presents one serious negative factor. At low temperatures, especially in the range of -40.degree. C. to -50.degree. C., it has been observed that the transmission loss in silicone clad-silica core optical fiber dramatically increases. In Applied Optics, 17, 3703 (1978), Yeung et al report that as the temperature of the silicone clad-silica core fiber decreases, the silicone cladding increases in density and refractive index much more rapidly than the silica core. Eventually, the refractive indices of the silicone and silica become equal. This precludes any further waveguiding effects due to complete frustration of total internal reflection at the core-cladding interface.
Thus, while silicone clad-silica core optical fibers represent an improvement over conventional fibers in radiation resistance, they are not effective at low temperatures. As many specifications for optical fiber systems require that they function at low temperature, there remains a need in the art for a radiation resistant optical fiber which is not as temperature sensitive as the recently studied silicone clad-silica core fibers.